When light strikes the retina of the eye, the photoreceptors in the retina (rods and cones) respond by generating a small electrical potential. The aggregate electrical potential representing the total response of all the rods and cones in the retina to a brief flash of light is an electroretinogram (hereinafter "ERG"). An ERG, together with other measurements, may be used by ophthalmologists to help determine the functional status of the retina in various conditions including congenital and acquired degenerative state, trauma (injuries), toxic states and inflammatory diseases.
An ERG may be obtained by photostimulating the retina. Conventionally, a light flash of several milliseconds duration is directed into the subject's eyes. The flash intensity must be variable so that the subject's ERG response can be evaluated throughout a broad range of adaptational states. In low ambient light conditions (dark adaptation), it is mainly rod photoreceptors which mediate visual sensation while in high ambient light conditions (light adaptation) cones dominate visual processes. Rod function is best evaluated with low intensity light flashes while cones respond to flashes of much greater intensity. Rod and cone function may also be differentiated by varying the chromatic properties of the light flash. Short wavelength (blue) stimuli are more effective in generating rod responses while long wavelength (red) stimuli produce mainly cone responses. Thus it is preferable to make provision for both variable flash intensity and chromatic filtering of the light stimulus.
The subject's ERG response is measured with a special corneal contact electrode. Usually, the response is displayed and stored for later analysis.
A typical prior art ERG stimulator comprises a reflective hemispherical bowl and a controllable light source for illuminating the bowl. The subject is fitted with the corneal contact electrode and then looks into the bowl as it is illuminated. Conventionally, three separate tests are used to measure the subject's ERG response. The first test is carried out in dark adaptation, following a specific period of time in dark ambient conditions sufficient to allow rod photoreceptor function to be fully expressed and well below the threshold at which cone function may occur. The eyes are then stimulated with a single light flash. The procedure is repeated, increasing the flash intensity in approximately three decibel (hereinafter "dB") steps over a maximum range of about 36 dB until an ERG with a peak of at least 50 microvolts (hereinafter "uV") is recorded. The 50 uV threshold is an arbitrary criterion threshold for photoreceptor response amplitudes broadly accepted within the clinical community. Following determination of the 50 uV threshold, responses are obtained at higher stimulus intensities, sufficient to evoke mixed rod and cone responses in order to evaluate more complex interactions between both classes of photoreceptors. The second test is carried out in light adaptation, following a brief period of time in ambient light of sufficient intensity to suppress rod function. A 50 uV threshold is determined for cone responses, followed by stimulation at higher intensities to elicit other components of the ERG having clinical significance in light adaptation. The third test is also carried out in light adaptation, a high intensity light source being presented at a stimulus frequency of about 30 Hertz (in contrast to the first and second tests in which the stimulus frequency is generally about 1 Hertz.) This higher frequency stimulus tests the temporal response characteristics of the cone photoreceptors, thus yielding further information regarding functional characteristics of these retinal cells.
The ERG represents the transient electrical responses of the retina to brief flashes of light, superimposed upon a more or less constant electrical potential originating in retinal structures, and present in both dark and light adaptation. The record of this "constant" or "standing" electrical potential is called the electro-oculogram (hereinafter "EOG"). Typically, the amplitude of this potential is considerably less in dark adaptation than in light adaptation. The ratio of the maximum amplitude in light adaptation to the minimum amplitude in dark adaptation is of clinical significance and is a useful adjunct to interpretation of the ERG in some disorders, as well as having importance in its own right in certain hereditary retinal degenerative conditions.
The "standing" potential referred to above is obtained by means of electrodes located horizontally adjacent to the eyes on the skin surface. Measurement of this potential is facilitated by instructing the subject to alternate his or her direction of gaze between two clearly indicated fixation targets placed 15.degree. horizontally eccentric to a reference point directly in front of the subject. Typically, the fixation targets are placed within the same hemispherical bowl used in the ERG test. Thus conditions exist such that the same apparatus used to present the appropriate background ambient light for the ERG can also be employed to present an appropriate background light ambience for the EOG test. An EOG test is commonly performed in three stages: a brief initial phase in which the subject views the fixation targets in light adaptive conditions, followed by a second phase in dark adaptation during which the subject views the dimly illuminated fixation targets and concluding with a third phase in light adaptation (the second and third phases each being of about 15 minutes duration).
The amplified electrical responses obtained during the three phases of the EOG procedure are commonly recorded on a slowly moving strip of chart paper over the duration of the test. At the conclusion of the test, the recording is analysed in order to determine those times at which the minimum and maximum responses in dark and light have occurred, followed by calculation of the ratio of those responses.
Photostimulation of the eye causes the generation of electrical potentials from the visual system in addition to the ERG and EOG. One other such potential, recorded from electrodes attached to the scalp of a test subject, is called the visually evoked potential (hereinafter "VEP") or visually evoked cortical potential ("VECP"). The VEP is a very small electrical signal and must be differentiated from other electrical activity occurring on the surface of the scalp by computer assisted signal processing procedures. An extremely wide variety of visual stimuli can be employed to elicit a VEP, one of the most commonly employed being a diffuse flash of light presented at some chosen temporal frequency within the field of vision. Stimulus frequencies generally range between 0.5 and 60 Hertz. The VEP largely reflects functional properties of the central cone-rich portion of the retina as well as conduction characteristics within the visual pathways leading to the visual cortex of the brain. The principal clinical application of the VEP relates to the evaluation of inflammatory and degenerative abnormalities of the optic nerves and posterior visual pathways.
Many problems have been encountered with prior art visual stimulators. The greatest difficulties have related to the source, regulation, measurement and calibration of the light flash stimulus. Most prior art visual stimulators have incorporated a stroboscopic gas discharge tube. Such sources have phipiologically undesirable spectral characteristics and stimulus energy is difficult to regulate. Measurements of the photometric and radiometric characteristics of the flash radiant energy are also difficult to obtain with such sources and in most cases flash energy declines to some extent as stimulus frequency increases. Prior art visual stimulator apparatus is often cumbersome and inflexible, making it difficult to test subjects in both upright and supine positions. Various difficulties have been encountered with respect to obtaining a permanent record of the responses obtained during ERG, EOG and VEP testing. The EOG in particular has been cumbersome to deal with due to the length of the continuously recorded potential.
The present invention addresses the foregoing disadvantages. For example, the light source of the preferred embodiment is an incandescent filament with a continuous spectrum having physiologically desirable chromatic characteristics. Energy emitted from this source is very stable. Of particular importance is the means used to interrupt the transmission of the light stimulus from the source to the subject's eyes and to regulate stimulus duration together with stimulus intensity. Means are provided which permit fine incremental control of stimulus intensity (attenuation) over a broad range of energy levels. Measurement of stimulus energy and luminance is provided and allows display of stimulus parameters in absolute radiometric and photometric units. The photodetector device employed in the preferred embodiment to obtain these measurements also mediates the self-calibration system incorporated in the preferred embodiment, ensuring a very high degree of control over the light stimulus reaching the subject's eyes. Provision is made for altering the chromatic characteristics of the stimulus in accordance with testing requirements. Another feature incorporated in the preferred embodiment is a means by which the response obtained from the ERG, EOG or VEP procedure can be transferred to a hard-copy record together with a reproduction of stimulus parameters on the same hard-copy record. This is of special significance with respect to the EOG response in that the results of a long recording procedure can be condensed into a compact hard-copy record.
The features described briefly above have been embodied in a device of sufficiently small size and flexibility to allow testing of subjects in a wide range of anatomical positions, and sufficiently mobile to allow testing in many clinical environments.